Epidemics of bacterial diseases are common in natural water streams and cultured fish populations. Predisposition to such outbreaks frequently is associated with poor water quality, organic loading of the aquatic environment, handling and transport of fish, marked temperature changes, hypoxia, and related stressful conditions. High concentrations of water-borne bacteria are normally found in ponds and aquaria. Many of these aquatic bacteria are opportunistic, facultative pathogens, being activated by an adverse environment or a debilitated host.
Aeromonas salmonicida is a gram-negative, non-motile, pigment-producing rod originally described as the cause of a septicemic disease of salmonid fish (furunculosis). It is also a serious pathogen of many other freshwater and marine fish and may produce high morbidity and mortality. In the acute form of the disease, hemorrhages are found in the fins, tail, muscles, gills, and internal organs. In more chronic forms, focal areas of swelling, hemorrhage, and tissue necrosis develop in the muscles. These lesions progress to crateriform abscesses discharging from the skin surface (furuncles). Carrier fish show no clinical signs of the disease while they are the ones which disseminate the disease to other populations of fish.
Aeromonas salmonicida, the bacterium responsible for furunculosis in fish, continues to be the cause of serious losses in cultured salmonids, and, despite the fact that some 40 years have elapsed since efforts were first undertaken to develop a vaccine to control the disease (Duff D. C. B., J. of Immunology, 1942, 44:87-94), a proven, mass-administered, anti-furunculosis vaccine without any latent period is still lacking. At the present time, the vaccine on the market does not give a good immunization and requires a long period of time before the active immunization takes place.
Avoidance is the most effective prevention since A. salmonicida is an obligatory fish pathogen. Fish and fish eggs should be obtained from sources known to be free of the disease. Infected stocks should be depopulated and wild-fish reservoirs eliminated.
A regular vaccine requires a latent period in order to be effective in protecting the administered patient, during which the patient's immune system is developing the antibodies and lymphocytes which would recognize the disease-causing agent.
Canadian Patent 915,086 discloses an antigenic composition useful for oral immunization of salmonids against furunculosis disease, a method of preparing the antigen and a method for administering the antigenic composition to fish. This oral immunization does not provide an immediate protection against furunculosis infection caused by A. salmonicida. The antigenic material administered in feed requires a latent period for the fish's immune system to provide an efficient protection against the infection.
According to the report of Aqua Health, a vaccine is usually efficient after a period of about 40 days at a temperature of around 10.degree. to 12.degree. C. and may require an even longer period if the temperature is lower than 10.degree. C. (AQUA HEALTH LIMITED, 1987, Report on Vibrio anguillarum and Aeromonas salmonicida, Charlottetown, P.E.I., Canada). This important delay between the administration of the vaccine and the appearance of immunity for the disease, may cause the stress of the administration itself to promote the disease before the fish are appropriately immunized (TURGEON Y., 1985, Report of `Ministere du Loisir, de la Chasse et de la Peche`, Quebec).
The antibacterial therapy, using antibiotics, provides to the fish an immediate protection against the infectious bacteria, but it inevitably results in a second generation of bacteria which is resistant to this antibacterial treatment. Even if this treatment has the advantage of procuring to the fish an instant protection against the pathogenic bacteria, the subsequent antibacterial treatment becomes complicated by the bacteria-inherited resistance. The use of antibiotics also requires a delay period, which is sometimes very long, before human consumption is possible.
The best way to protect fish against A. salmonicida would be to have a product or a method which would combine the advantages of the antibacterial treatment with the advantages of the vaccine without having their respective disadvantages.
It would be highly desirable to have a means to effectively immunize fish against A. salmonicida for a considerably long period of time and without having a latent period wherein the fish are vulnerable.
Moreover, it would also be highly desirable to have a passive immunization means which would readily protect the fish against this bacteria upon their administration.